Introduction Measured fluxes of volcanic SO emissions are important in the 2

evaluation of the state of activity of a particular volcano. Changes in SO emission rates from an established baseline often indicate a 2

coincident change in eruptive activity. SO has a characteristic 2

absorption pattern in the ultraviolet range of radiation wavelengths; remote, ground-based measurements can easily be accomplished by measuring the degree of UV absorption by a volcanic plume. Current methods of monitoring volcanic SO 2

(e.g., COSPEC, FLYSPEC) are limited to making cross-sectional scans of the plume; a simple one-dimensional profile of plume concentrations is not necessarily representative of the entire plume. These scans are limited to a temporal resolution of one every few minutes at best. Derivation of accurate SO fluxes using 2

such a method may be further complicated by changes to the plume between the emission and measurement sites, including plume speed variations, plume dispersion, and loss of SO from 2

the plume, as with dry deposition or conversion to sulfate aerosols, none of which are detectable by a single scan through the plume.

In an effort to improve monitoring of passively degassed volcanic SO , we have constructed and tested a digital camera 2

for collecting two-dimensional images of volcanic plumes.

Series of images from Fuego and Pacaya volcanoes, Guatemala, Masaya volcano, Nicaragua, and Colima and Popocatépetl volcanoes, México, were collected in January, March, and November 2008, respectively. Except the Mexican volcanoes, at each volcano one or more broadband seismometers recorded activity simultaneous to the UV camera. At Fuego and Pacaya volcanoes, a FLYSPEC UV spectrometer pointed at the active vent recorded SO output continuously, and 2

at Masaya volcano, one day of UV camera measurements was accompanied by 13 FLYSPEC road traverses.

Results highlight not only the much improved temporal resolution of the camera over more traditional SO measurement 2

techniques (e.g., FLYSPEC, mini-DOAS), but also the potential for recognition of very fine-scale variations in volcanic SO output. 2

The combination of both these factors shows the potential for detailed comparisons of SO emission rates with other geophysical 2

datasets.

The UV Camera

The equipment consists of:

- an Apogee U6 Alta digital camera with the Kodak KAF-1001E-2 CCD

- a UV CostalOpt SLR lens- two Andover Optics bandpass filters, centered at

307 nm and 326 nm- calibration cells from Resonance, Ltd., with SO2

Concentration-pathlengths of 90 and 270 ppm•m

- a filter wheel

The UV camera relies, as UV spectrometers (e.g., COSPEC, mini-DOAS) do, on Beer’s Law:

A=log10(I /I)0

where A is absorbance, I is the light intensity before 0

passing through the plume, and I is the light intensity after passing through the plume. Plume absorbance values in plume images are scaled to absorbances of the background sky and calibration cells in separate images.

2008

V11C-2074AGU FALL MEETING15-19 DECEMBER - SAN FRANCISCO

Figure 1

COMPARISON OF HIGH TEMPORAL RESOLUTION SO EMISSION RATES AND2GEOPHYSICAL DATA AT CENTRAL AMERICAN VOLCANOES

COMPARISON OF HIGH TEMPORAL RESOLUTION SO EMISSION RATES AND2GEOPHYSICAL DATA AT CENTRAL AMERICAN VOLCANOES

1 1 1 1 1,2Patricia A. Nadeau (panadeau@mtu.edu) , Gregory P. Waite , Jose L. Palma , Marika P. Dalton , and I. Matthew Watson

1 2Department of Geological and Mining Engineering and Sciences, Michigan Technological University, Houghton, MI 49931, USA Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, UK

http://www.geo.mtu.edu/rs4hazards/index.html

COMPARISON OF HIGH TEMPORAL RESOLUTION SO EMISSION RATES AND2GEOPHYSICAL DATA AT CENTRAL AMERICAN VOLCANOES

COMPARISON OF HIGH TEMPORAL RESOLUTION SO EMISSION RATES AND2GEOPHYSICAL DATA AT CENTRAL AMERICAN VOLCANOES

COMPARISON OF HIGH TEMPORAL RESOLUTION SO EMISSION RATES AND2GEOPHYSICAL DATA AT CENTRAL AMERICAN VOLCANOES

Methodology

Images are collected at a rate of approximately 1 or 2 per second, depending on the exposure time necessary at ambient light levels.

Camera images suffer from vignetting, or dark corners and edges, as a result of aperture settings and lens design, in the same manner as standard visible-light cameras. To ensure that darkened edges are not erroneously deemed SO in the frame, all plume images 2

are divided by a clear sky, or background, image. The vignetting in the clear sky image compensates for most of the vignetting in the plume image while preserving the plume information.

Following vignetting corrections and conversion of raw imagery to maps of SO 2

concentration-pathlength through the application of Beer’s Law (Figure 2a), a profile near the vent and perpendicular to the direction of plume motion is sampled from each image (Figure 2b). Corrections for background drift are made (Figure 2c) and the plume is isolated from background noise. Each cropped profile (Figure 2d) is then integrated over the length of the profile to yield an integrated column amount (ICA) of SO . Profiles along the length of the plume are 2

made for each image (Figure 2e), and successive profiles iteratively fitted to each other to determine a spatial lag that

2maximizes R . This lag, divided by the temporal resolution of the images gives a vector of plume speeds for each pair of images. Subsequent multiplication of ICAs by the plume speed and a unit conversion factor gives variations in SO emission rate on the 2

order of seconds.

Raw plume and cal cell images are flattened using a clear sky image. On the calibration cell image, clear sky, the low cell, and the high cell are isolated, and their average values used to determine a linear calibration curve, which is then used to convert raw plume imagery to SO 2

concentration pathlength in ppm•m.

User input in the processing code allows for the selection of profile endpoints by simply clicking on the image. One profile may be used for an entire image sequence, or different profiles may be used for subsets of the sequence depending on plume direction. Generally, a profile as close to the vent as possible is used (black line, above).

Profiles from the sequence are plotted adjacent to each other to inspect for background drift due to changes in illumination and exposure time. Average background values for each profile (in white box, above) are subtracted from all values in that profile to account for such background drift and to ensure a background close to zero.

Once the background drift has been accounted for and the data cropped to include as little background as possible, profiles are integrated over their length in meters (derived from camera CCD resolution, distance of camera from the plume, and lens focal length) to yield an Integrated Column Amount (ICA) of SO 2

for each image in the sequence.

Profiles from the same location in sequential images are shifted

2incrementally, with R calculated for each new alignment of the data. The lag that

2coincides with the maximum R represents the distance over which the plume has moved from one image to the next, giving plume speed along the profile. Derived plume speeds multiplied by ICAs gives a time series of SO emission rates.2

In a similar manner as in figure 2b, interactive programming allows for user selection of a representative profile along the direction of propagation of the plume. The image is then sampled over the profile line for each image in the series. Also, using header information for the first and last image in an uninterrupted subset of the main series, a temporal resolution is established.

100 300 500

ppm•m

Figure 2a 2b

2f2e2d

2c

AcknowledgementsThis work was supported by U.S. National Science Foundation PIRE/OISE 0530109. Logistical support in Central America was provided by INSIVUMEH, CONRED, Parque Nacional Volcan Masaya, and the CCVG workshop. Field work also benefitted from the help of many field assistants, and equipment loaned by Simon Fraser University.

Future WorkCurrent and planned work to improve and develop UV camera SO 2

emission rate measurements includes:

- Evaluation of camera response to a wider range of SO2

concentrations using higher concentration calibration cells - Integration of data taken with a filter centered at 326 nm to

improve isolation of SO from other plume constituents2

- Further improvement of plume speed derivation algorithm

Plans for utilization of camera-derived SO data in developing models 2

of behavior at these, and other, volcanoes include:

- Addition of infrasonic data to the SO dataset2

- Deployment of an extensive seismo-acoustic array at Fuego volcano for the examination of distinct event types and source mechanisms as they relate to fluctuations in SO emissions 2

(Figure 7)

Proposed deployment of three, five-station seismic arrays (red triangles) and locations of UV-camera imaging locations (blue triangles). The seismic arrays are optimized for characterization of tremor expected to be the dominant seismic signal. The UV camera locations will provide views of the plume from distances of 1 - 1.5 km. The scale bar is 2 km.Figure 7

Small-scale variations on the order of 30 seconds at Masaya (Figure 5, left) may be due to wind eddies and a profile farther away from a larger vent rather than true variations in SO emission rate. Similar 2

variations at Fuego (Figure 5, right), where the profile samples much closer to a smaller vent, may be more likely to represent real differences in volcanic SO output.2

- While FLYSPEC temporal resolution is better than the UV camera in this instance, the FLYSPEC was stationary, pointing at the vent. While relative changes in SO may be accurate, fluxes cannot be derived, and depending on the width of the 2

plume and location of puffs, some emissions may be missed by the FLYSPEC.

- Note extreme difference in temporal resolution between typical FLYSPEC traverses and UV camera data.- FLYSPEC traverses were performed ~5km from vent; both data sets are reported at time of acquisition, so a lag time exists for a given parcel of SO between the two measurement sites.2

- Temporal resolution of the UV camera data still lags somewhat behind that of other geophysical datasets like RSAM. Gaps in the SO emission data sequences at left represent pauses in data collection in order to take back-ground and 2

calibration cell images. Autofocus cameras and background proxies will reduce the length of the gaps in the future.

* Plume speeds in Figures 3a-e were determined by visual inspection of the movement of individual puffs through subsets of the full series of images. The mean of the plume speed from 3 subsets for each series was then calculated and applied as a single plume speed for the entire day’s data. Plume speeds in Figure 5 were derived with the image-to-image plume speed methodology described in Figure 2f.

Figure 5

Fuego volcano:

- The UV camera-derived SO data (Figure 6, bottom) from Fuego appears to indicate that there 2

may be a cyclic nature to the magnitude of emission rates from the volcano. For over the first 45 minutes of the data sequence, emission rates vary from below 0.5 kg/s to nearly 3.5 kg/s over time scales of about 10 minutes.

- 10-second RSAM and SSAM (Figure 6, top) indicate the presence of high-frequency seismic events of various magniudes (e.g., at times ~15:20, ~15:30, ~16:00, and ~16:20). The events appear, in some cases, to be preceded by a decrease in SO emission rate.2

- Of note is the seismic event dominated by lower frequencies (@ ~16:10), which, unlike higher-frequency events, does not appear to be preceded by a significant drop in SO emissions.2

3b

Masaya - 03/16/08, plume speed = 10.30 m/s

Figure 3a

Masaya - 03/19/08, plume speed = 10.04 m/s

3c

Pacaya - 01/07/08, plume speed = 5.02

Figure 6

Fuego01/15/08

Results

Popocatépetl - 11/14/08, plume speed = 7.12 m/s

3d

80000

85000

90000

95000

100000

19:32:00 19:34:00 19:36:00 19:38:00

- Example of puffing from vent at Popocatépetl volcano, Mexico- Short time-scale variations in the size and opacity of the visible plume during measurements also seem apparent in SO emission rate time series (see zoomed-in portion of data, right), interpreted to be puffing2

- Example of sharp increase in SO emissions upon eruption onset (red line) at Colima volcano, visible in images (Figure 4) only 3 minutes apart. 2

- Note that pre-eruptive SO emissions are at levels undetectable by the UV camera (Figure 4, left).2

- Eruption plume was ashy, but SO retrieval algorithm assumes ash to be SO . Therefore SO amounts for eruptions like this are likely overestimates.2 2 2

3e

Colima- 11/22/08, plume speed = 3.87 m/s

Figure 4